Motivation for this change

With this PR, a script can be triggered when an OpenVPN connection is made that will configure the DNS for systemd-resolved. Short version: if you need to do DNS over an OpenVPN tunnel that you run directly by invoking openvpn, you need this.

There is no change to the openvpn binary itself, so there is no risk of breaking anything.

Without this PR:

Link 20 (tun0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

With this PR:

Link 20 (tun0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
  Current DNS Server: 10.10.1.63
         DNS Servers: 10.10.1.63
          DNS Domain: some.other.domain.tld

Cc: @viric

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.